Clean up unneeded variable. Use filepath.Join correctly.

The variable goroot was not being used anywhere else, except as part of
gorootSrcPkg. It can be removed.
Also, filepath.Join accepts individual path elements and cleans the
result, so I've split "src/pkg" into two path elements.

go13.go

@@ -2,6 +2,9 @@
 
 package gotool
 
-import "path/filepath"
+import (
+	"path/filepath"
+	"runtime"
+)
 
-var gorootSrcPkg = filepath.Join(goroot, "src/pkg")
+var gorootSrcPkg = filepath.Join(runtime.GOROOT(), "src", "pkg")

go14.go

@@ -2,6 +2,9 @@
 
 package gotool
 
-import "path/filepath"
+import (
+	"path/filepath"
+	"runtime"
+)
 
-var gorootSrcPkg = filepath.Join(goroot, "src")
+var gorootSrcPkg = filepath.Join(runtime.GOROOT(), "src")

match.go

@@ -37,12 +37,9 @@ import (
 	"path"
 	"path/filepath"
 	"regexp"
-	"runtime"
 	"strings"
 )
 
-var goroot = filepath.Clean(runtime.GOROOT())
-
 var buildContext = build.Default
 
 // matchPattern(pattern)(name) reports whether